A baitwell for a sailing boat

While I am waiting for an engineer to finish a little job on the top of the shaft of my new rudder, I've been working on making a baitwell. I want to fish using live bait, and sandeels and small mackerel are the best all round bait available. They're both easy to catch on the Skerries, a sandbank just out of Dartmouth. What I want to do is fish for my bait on the Skerries before sailing to where the bigger fish live.

I built a baitwell, and planned to use a pump that currently powers the shower to pump the seawater into it. Unfortunately, I fell into the trap again of copying methods suitable to boats with engines. Boats with engines are boats with a plentiful electricity supply. They don't need to consider the efficiency of things like water pumps. They can pump water up from sea level up to say two metres, and allow the overflow to drain back into the sea. But when I uninstalled my shower pump, I was disappointed to read on the pump that it typically uses 8 amps at 12v. Which is far more than any other equipment I run. My battery would drain in a few hours. The battery I use to run the fishfinder, laptop, navigation lights and other lights, and the self-steering gear. The only way I could run a pump like that would be to also run a generator. I don't fancy the expense, the work of installing it, or the noise.

But I need a baitwell that I can keep bait in for several days. Sandeels are much easier to catch on the Skerries that out over some deep water wreck. I did some thinking...

I understand that lifting sea water to a height of a couple of metres is going to consume a fair amount of power. But what I really want to do is simply change the water in the baitwell  tank. If I balance the work of lifting the fresh water into the with the work of dropping the water back into the sea, I'll need far less power. I've found a pump designed to run for long periods (unlike my shower pump designed to run for no more than 20 minutes at a time) using low power - around 1 amp. It's a circulating pump, and now I need to make a baitwell that uses the work of the water dropping back to the sea.

To do this, I need a baitwell that I can close off with an air-tight lid, and run the stale water through a pipe down to the sea level. With the lid closed, the pump is just circulating water from the sea through the tank, using very little power. With the lid open, the pump won't have enough power to change the water in the tank. The tank will need to be filled using a bucket, and while I'm fishing for bait, or getting bait out of the tank to use, I'll have to occasionally add a bucket of fresh sea water to the tank. But this is the only way I can see of keeping bait alive for days at a time, using only a small amount of electricity.

So the task for today is to find a suitable container that has an airtight lid. First stop, Newton Abbot dump!